Many small PMDC motors are used in automobiles and are subject to harsh conditions. A fuel pump motor may be mounted on the engine block and subject to very severe G forces, sometimes up to 100 G. Forces of this magnitude can cause relative movement of the laminated armature stack and the commutator on the motor shaft, which may strain or break the armature winding wire.
The armature stack is typically provided with a structural coating such as epoxy resin. The coating may be allowed to flow along the shaft to the commutator base. However, the coating thus formed on the shaft tends to crack as the shaft flexes and thus may crumble or flake and allow the commutator to creep relative to the stack.